Les Bell has released (Jan 2012) his course material "Introduction to OS/2 Warp Programming" under the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported license. The course had been released with its original files, OOXML, ODF, PDF version and lab exercises.

Well, most of my previous post just mentions straightforward facts (but tell me, did you remember that Symbian and GEM* can be had under OSS licenses, while sort of wishing there about some 2nd youth for OS/2?), it's grounded in reality (and IMHO using it as a guide to judge new possibilities is useful; for one, it's generally nicer to allocate scarce resources where it makes most sense)

*they also ~"can still perform the basic things any other operating system can perform... and it is already written, just needs some "help" to modernize it"

Just came back from a vacation in the Philippines, while on Cebu I came across an ATM that is still using OS/2 Warp, the ATM was far from old but it shows how solid the OS still is today. Why IBM will not release it is beyond me since they have said they have no intention of doing anything with it.

Which wasn't that unusual ...OS/2 (initially at least) wasn't far from it, IIRC. Plus: isn't modularity good, isn't random DE + Linux just like that?
(though I associate GEM more as "the" OS of 16bit Atari)

Anyway, I'm not sure if that different - OK, GEM is sort of not relevant for a long time. But whatever its technical characteristics are, it was open sourced many years ago - when, at the end of 90s, it was approximately as disconnected from contemporary developments as OS/2 would be now... perhaps not that different category.

With Symbian... there aren't that many ~mature open source mobile OS - but seemingly it took only the earlier (open) existence of Android to steal virtually whole community momentum, and make other (not only Symbian) not very viable.

Yeah (@edit), depends which aspect we look at (so we might show pretty much anything ). I could be revealing my personal bias, my views that came after some years of toying with various niche tech (also operating systems) - namely how it perhaps matters more what we do via technology, instead of with the technology itself.

Germany had always been an Atari stronghold so it was not surprising that most Atari software development was happening here. One of the projects to create a new AES was initiated by programmer Martin Osieka. [...] While enthusiastic Atari magazines had stated that over 50 developers had teamed up helping Osieka with the project, things came to a grinding halt when his Atari machine broke.

Srsly, WTH? :p

And I think I stumbled once at least at one OS/2 reimplementation "project" (change log didn't look encouraging) in the style of Haiku. It didn't look very sustainable.
Also, add RISC OS to that short list, it's also kinda being open sourced, I think, some limited revival.
Still, dead or dying (or at least without forward momentum); don't expect comebacks after "just a little work" (that's usually the sentiment, it seems) as OSS - it would have to be a major undertaking (and most likely having problems attracting enough competent devs - I guess many of them also not seeing much point to direct their efforts where they would be felt by very few)